Nitrogen solutions represent an important class of fertilizers. A commercially popular nitrogen fertilizer solution is made from urea and ammonium nitrate, often referred to as UAN. The UAN does not need to be kept under pressure, and can be applied directly for agricultural purposes.
The production of UAN solutions is straightforward, comprising blending urea solution, ammonium nitrate solution and any additional water in a mixing tank, in either a batch or a continuous process. Ammonia is sometimes also added to adjust the pH. Mixtures of ammonium nitrate and urea have much greater solubility as compared to that of either material alone. The UAN can be made with as little as 20% by weight water, but more typically contains 30-40% water by weight. The economics of such solutions are relatively attractive in comparison to solids because evaporation is decreased and granulation, drying and conditioning are not necessary.
One problem that has been persistent in the production, storage, transportation and use of UAN has been that the UAN liquid is extremely corrosive to steel piping systems. Without adequate corrosion inhibition, UAN solutions in ferrous tanks or piping systems will become colored within a matter of hours, usually orange or reddish indicating iron corrosion. Filmers have been used to inhibit corrosion, but these suffer from numerous problems. Where the filmer is poorly dispersed in the UAN solution, sludge can be seen as a ring on sample bottles and forms an undesirable buildup in piping and tank bottoms and walls. The use of a dispersant to avoid the sludge problem can cause the UAN solution to foam. Anti-foam additives generally become less effective with time, so the foaming problem can be addressed initially but often will become a serious problem before or during application of the UAN solution. Also, filmers tend to be depleted as the UAN solutions are transferred through a pipeline or from tank to tank since the filmer coats the pipeline or tank surface to protect it from the UAN solution. As the filmer is used up, there is less and less left in the UAN solution for inhibiting corrosion. After the UAN solution has seen enough metal surfaces to use up all of the inhibitor, it suddenly becomes uninhibited and highly corrosive.
The selection of corrosion inhibitors for liquid fertilizer solutions is made more difficult by the presence of environmental considerations. Since the fertilizer solutions are applied to crops, for example, they must be free of compounds which are toxic to the crops being fertilized, and must also facilitate compliance with industrial hygiene standards for the personnel applying the fertilizer. Thus, fluoride compounds, as one example, are undesirable in UAN solutions because they are generally agrotoxins.
Other corrosion inhibitors which are sometimes used in boilers and cooling towers, for example, are incompatible with the relatively more severely corrosive UAN. Borate, for example, a common rust inhibitor, is not sufficiently soluble in UAN and precipitates.